1. Field of the Invention
The present invention relates to a powertrain of an automatic transmission. More particularly, the present invention relates to a powertrain of an automatic transmission that enables eight forward speeds and two reverse speeds by combining three simple planetary gear sets and six frictional elements.
2. Description of Related Art
A typical shift mechanism of an automatic transmission utilizes a combination of a plurality of planetary gear sets. A gear train of such an automatic transmission that includes the plurality of planetary gear sets changes rotating speed and torque received from a torque converter of the automatic transmission, and accordingly changes and transmits the changed torque to an output shaft.
It is well known that when a transmission realizes a greater number of shift speeds, speed ratios of the transmission can be more optimally designed and therefore a vehicle can have better fuel mileage and better performance. For that reason, an automatic transmission that enables more shift speeds is under constant investigation.
In addition, with the same number of speeds, features of a gear train such as durability, efficiency in power transmission, and size depend a lot on the layout of combined planetary gear sets. Therefore, designs for a combining structure of a gear train are also under constant investigation.
A manual transmission that has too many speeds causes inconvenience of excessively frequent shifting operations to a driver. Therefore, the positive features of more shift-speeds are more important for automatic transmissions because an automatic transmission automatically controls shifting operations basically without needing manual operation.
The information disclosed in this Background of the Invention section is only for enhancement of understanding of the general background of the invention and should not be taken as an acknowledgement or any form of suggestion that this information forms the prior art already known to a person skilled in the art.